The present invention relates in general to variable voltage converters in electric drive systems for electrified vehicles, and more specifically to duty cycle modulation and use of selectable dead time insertion for power switching devices in a converter to achieve a greater range of boost ratios when operating in a boost mode.
Electric vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), use inverter-driven electric machines to provide traction torque. A typical electric drive system may include a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main DC linking capacitor. A 3-phase motor inverter is connected between the main buses and a traction motor in order to convert the DC bus power to an AC voltage that is coupled to the windings of the motor to propel the vehicle. The motor is driven by the vehicle wheels and may be used to deliver electrical power to charge the battery during regenerative braking of the vehicle. Another 3-phase inverter connects a generator to the DC bus. The generator may be driven by an internal combustion engine to charge the battery. During charging, the VVC converts the main bus voltage to a voltage appropriate for charging the DC battery pack.
Using the appropriate modulation of the power switches, a VVC can boost a direct current voltage provided by the battery to a higher voltage to drive the motor at an improved level of vehicle performance. When used to boost a voltage from an input side to an output side, the converter is referred to as a boost converter. In the field of hybrid vehicles, it is common practice to charge a battery through regenerative braking, in which the mechanical energy of the wheels is converted to electrical energy by a generator, or by a motor operating as a generator, and provided to the battery via the VVC. The VVC can also operate in a pass-through mode in which transient current flows to/from the battery side from/to the inverter side, with no boost in voltage.
The VVC includes upper and lower transistor switching devices (e.g., insulated gate bipolar transistors, IGBTs) have an intermediate junction connected to the source battery via an inductor. The switching devices are connected in series between the positive and negative DC buses. An electronic controller provides switching signals (i.e., gate signals) to turn the switching devices on and off according to a modulation scheme that provides the desired VVC mode. Pulse width modulation is typically used to control the stepping up of a voltage by the VVC, wherein a duty cycle of the switching signals can be varied in order to regulate the VVC voltage to a desired magnitude. The “boost ratio” of the VVC is defined as the ratio of the output voltage to the input voltage. In pass-through mode, the boost ratio is one. Otherwise, the boost ratio is greater than one (i.e., the voltage on the inverter side of the VVC is higher than on the battery side of the VVC).
To avoid a short circuit across the DC link, it is important that the upper and lower devices not be conducting (i.e., turned-on) simultaneously. A short time interval (known as dead-time) is typically inserted at any transitions in the switching signals during which both the upper and lower switching devices are turned off in order to prevent such shoot-through. The insertion of dead time, however, changes the effective duty cycle of the PWM switching signals. During regeneration operation, for example, the dead time may increase the minimum achievable boost ratio significantly above the value of 1.0 (e.g., as much as 1.25 at a high PWM switching frequency around 20 kHz). The unavailability of a lower boost ratio may lead to a higher than desired voltage on the main DC bus or to a forced used of the pass-through mode which results in lower torque production by the motor due to a lower than desired voltage on the main DC bus.